Also the ReplaySafeSplit and related contracts were due to the ETH/ETC split, you had to move your coins to be safe.
I see no evidence of a "mixer" being the cause.
I don't understand how the author could group all temporary addresses, see the top inputs/outputs as all exchanges, and then claim some nefarious mixer was responsible.
Ethereum should allow sending directly to 0xAddr#input where input could be kind of tag that is used to identify you.
Deposit addresses aren't shared across accounts. The re-shuffling of the coins is for the exchanges security and accounting
Neither requires re-shuffling. It's just for convenience and what be even more convenient if every blockchain tx included who they are trying to deposit to providing inputdata even for regular transfers. Like a tag, "these 10 eth for personal_Hash=abc123"
So I'm pretty certain even if it were supported, individual addresses would still be the better choice due to ease of use.
Normally this is used for function calls to contracts. An exchange could make a contract that lets the user put in their userId when sending eth, and then everybody would be sending to the same address while still being identifiable. It would even be possible to reject deposits which do not include the userId, or which have an unknown userId.
I think the main reason exchanges don't do this is that they deal with lots of cryptocurrencies, so they use the simplest method that works for all of them: just make a unique deposit address for each user.